envsetup: do not print an error if complete fails

complete is a bashism; so it will fail on other shells. Avoid printing
an error if that is the case.

Change-Id: Id6d6311792f409cc3a697c7a2bb003863f1afe60
diff --git a/envsetup.sh b/envsetup.sh
index b1c6056..b3066a9 100644
--- a/envsetup.sh
+++ b/envsetup.sh
@@ -654,7 +654,7 @@
     COMPREPLY=( $(compgen -W "${LUNCH_MENU_CHOICES[*]}" -- ${cur}) )
     return 0
 }
-complete -F _lunch lunch
+complete -F _lunch lunch 2>/dev/null
 
 # Configures the build to build unbundled apps.
 # Run tapas with one or more app names (from LOCAL_PACKAGE_NAME)